High speed running of tires, e.g. higher than 200 km/h generates remarkable centrifugal forces at the tires tread band due to rotation thereof.
Said centrifugal forces cause the tire tread band to swell outwardly resulting in the lifting of the tires tread band in the radial direction. This is known as “lifting phenomenon”.
This phenomenon should be suitably controlled and limited as much as possible since it negatively affects the tires behaviour.
For instance, since vehicle electronic systems (e.g. Anti-lock Braking System (ABS), Electronic Stability Program (ESP), traction distribution on the four driving wheels) are traditionally correlated to the variation of the wheel rolling height and are set to a predetermined range thereof, in the case the lifting phenomenon gives rise to an important swelling of the tire—so that the wheel rolling height falls away from said range—a correct functioning of the abovementioned vehicle electronic systems is no more guaranteed.
Furthermore, in the case the lifting phenomenon is not suitably controlled and limited, an important and different variation of the wheel rolling height may occur in the tires of the same vehicle so that the latter may result to have tires operating differently from each other.
Moreover, in the case the tire deformation due to the lifting phenomenon is considerably high, a plurality of further drawbacks may occur.
For instance, the lifting in the radial direction of the crossed belt layers may arise, especially in correspondence of the axial edges thereof, thereby causing the detachment of the belt layers from the carcass; an uneven wear of the tread band and thus a remarkable decrease of the durability thereof at high speeds may occur; undesired vibrations of the tire resulting in negatively affecting the ride comfort and remarkably increasing the noisiness of the tire at high speeds may be promoted.
In order to at least partially solve the abovementioned drawbacks caused by the lifting phenomenon, a belt layer is generally positioned radially external to the crossed belt layers in order to constrain the latter so as to limit the lifting thereof. Generally, said belt layer is provided with low elastic modulus organic fiber cords, e.g. nylon cords, or high elastic modulus organic fiber cords, e.g. aromatic polyamide cords, which are disposed in a substantially circumferential direction with respect to the equatorial plane of the tire.
However, as known in the art (see, for example, European Patent EP 335,588 below disclosed), the use of low elastic modulus organic fiber cords may not successfully prevent said lifting phenomenon, in particular at very high speed.
On the other end, the use of high elastic modulus organic fiber cords may cause some drawbacks during tire manufacturing. As a matter of fact, when the green tire is mounted in the vulcanization mould and the internal pressure is applied, the essential swelling of the green tire by the internal pressure is reduced. This is because the resistance to stretch of said cords is excessively high and therefore the tire may not correctly expand into the mould to give the necessary force to press the tread band and structure of the tire against the inner face of the tire mould. This often causes defects in the finished tire after vulcanization and moulding. Consequently, the tire high speed performances such as, for example, tire steering stability, and the tire high speed durability, may be negatively affected.
Attempts have been made in the art to overcome the above reported drawbacks.
For example, European Patent EP 335,588 discloses a tire, particularly suitable for high speed passenger cars, comprising a band disposed radially outside the tire belt, said band comprising a ply composed of at least one cord wound spirally and continuously in the circumferential direction of the tyre at 0 to 3 degrees to the equator of the tire. The cord of said ply is a hybrid cord comprising a high elastic modulus yarn and a low elastic modulus yarn twisted together, the hybrid cord having a low elastic modulus in a low elastic modulus zone between zero elongation and a predetermined specific elongation in the range of 2-7% and a high elastic modulus in a high elastic modulus zone above said predetermined specific elongation of the cord. The low and the high elastic moduli change at a transitional point derived from the load elongation curve of the hybrid cord, being the intersecting point of a line orthogonal to the elongation axis passing through the intersection of the tangent to the elongation curve at zero elongation and the tangent to the elongation curve at the break point. The abovementioned tire is said to have improved high speed durability, as well as reduced noisiness.
U.S. Pat. No. 5,558,144 relates to a pneumatic radial tire containing a non-metallic cord breaker belt disposed radially outside a carcass, and a jointless band belt disposed radially outside the breaker belt, said band belt being made of at least one hybrid cord wound spirally and continuously in the circumferential direction of the tire at an angle of 0 to 3 degrees with respect to the tire equator. The hybrid cord includes a low elastic modulus thread and a high elastic modulus thread which are finally-twisted together, said low modulus thread having at least one low modulus fiber, first-twisted and having an elastic modulus of not more than 2000 kgf/mm2 and said high modulus thread having at least one high modulus fiber first-twisted and having an elastic modulus of not less than 3000 kgf/mm2. In the band belt ply, the count (E) of the hybrid cord per 5 cm width, the stress (F1) in kgf of the hybrid cord at 2% elongation, and the stress (F2) in kgf of the hybrid cord at 6% elongation satisfies the following relationships: F1×E<60; and F2×E>150. The abovementioned tire is said to show the following characteristics: decreasing change of tire diameter, improved durability, improvement of the total tire performance.
United States Patent Application US 2004/0265581 relates to a hybrid cable having a ratio of final tangent modulus vs initial tangent modulus greater than 10. Preferably, said hybrid cable comprises a textile core of an initial modulus of less than 900 cN/tex and a textile wrap of an initial modulus greater than 1300 cN/tex which is wound on said core. A process for obtaining said hybrid cable, as well as a composite fabric usable in a tire incorporating said cable, is also disclosed. Moreover, a tire and a mounted assembly which each incorporates such a composite fabric is also disclosed. The abovementioned hybrid cable is said to be advantageously used in both passenger-vehicle tires and heavy-vehicle tires. It is also said that said hybrid cable allow to improve the endurance at high speed (typically greater than 120 km/h) of the crown reinforcement of tire the hooping crown ply of which is reinforced by these cables.
United States Patent Application US 2004/0221937 relates to a belt bandage for a vehicle pneumatic tire that includes a ply having strength carriers embedded in an unvulcanized rubber mixture. The strength carriers run essentially parallel to one another. Each strength carrier includes a hybrid cord that has a first twisted yarn having a high modulus of elasticity of at least approximately 25000 N/mm2 and a second twisted yarn having a low modulus of elasticity which is no greater than approximately 15000 N/mm2. The first and second twisted yarns are final-twisted together. A cord count E in cords per 5 cm ply widths, a force F1 at 2% elongation of each hybrid cord, and a force F2 at 6% elongation of each hybrid cord satisfies the following relationships:F1×E≧approximately 600 N,F2×E>approximately 1500 N.A vehicle pneumatic tire containing said belt bandage is also disclosed. The abovementioned belt bandage is said to ensure a very good high-speed durability of the tire and, in addition, an improved abrasion behaviour of the tire, i.e. a reduced abrasion in particular in the speed range of up approximately 200 km/h, and an increased long-term durability.
U.S. Pat. No. 5,688,594 relates to a hybrid yarn consisting of at least two varieties of filaments, at least one variety (A) having a lower heat shrinkage and at least one variety (B) having a higher heat shrinkage than the rest of the filaments of the hybrid yarn, wherein: the first variety (A) of filaments has a dry heat shrinkage maximum of below 7.5%; the second variety (B) of filaments has a dry heat shrinkage maximum of above 10%, and its dry heat shrinkage tension maximum is so large that the total shrinkage force of the proportion of the second variety of filaments is sufficient to force the lower-shrinking filaments present to undergo crimping; the optionally present, further filament varieties (C) have dry heat shrinkage maxima within the range from 2% to 200%; and at least one of the filament varieties (B) and/or (C) is a thermoplastic filament whose melting point is at least 10° C., preferably 20° C. to 100° C., in particular 30° C. to 70° C., below the melting point of the lower-shrinking component of the hybrid yarn. Also described is a process for producing the hybrid yarn and the use of the hybrid yarn for producing permanent deformation capable textile sheet materials and fiber reinforced shaped articles. No mention is made about use of the abovementioned hybrid yarn in tires.